Vat Dye

What is vat dye?

The wooden container that was initially used for the reduction and application of vat dyes gave rise to the word “vat.” Vat dyes are created naturally, not artificially. Vat dyeing is frequently used to color cellulose materials like cotton. Due to its insoluble nature in water, vat dye cannot be applied directly and must be vatted. However, by reducing them in an alkaline solution, they can become soluble and attach to the strands of fabric. The dye is transformed back into its insoluble state by further oxidation or exposure to air. The first vat dye was indigo. These dyes for cotton, linen, and viscose are the swiftest available. To dye other materials like wool, nylon, polyester, and acrylic, they are combined with mordants. However, of all dyes, the fastness properties of vat dyes are the best. The generic name for the dye comes from vatting. The term “vat dyes” refers to naturally acquired coloring elements that have been preserved in wooden vats since ancient times and made soluble in the vat through the fermentation process.

Why So Called Vat Dye?

Vat translates to “vessel” in English. From vatting, the colors get their name. It is known as “vat dye” because the vat dyes are naturally colored materials that are maintained in wooden vats and made solubilized by fermentation. They are utilized in a specific type of dyebath where the dye is converted into a soluble form using a potent reducing agent, such as hydrosulphite. Due to the intractable nature of the vat dyes, vatting is necessary before usage. The best fastness qualities of any dye are found in this one. The best overall fastness features for dyeing cellulosic materials, such as washing fastness, light fastness, and chlorine fastness, are found in vat dyes, which are among the most expensive dyes employed in this process.

What is Vatting?

The technique of vatting involves turning insoluble vat dyes into their soluble form. Because evenly dyeing the substrate in an aqueous medium is what we ultimately want to achieve. Additionally, the soluble form of the vat dye exhibits substantivity toward cellulose while the insoluble form does not. We can turn the insoluble vat dye into its soluble leuco form with the use of a suitable reducing agent and alkali. Color generally changes as a result of this.

Pretreatment for Vat Dyeing:

Pretreatment is crucial before vat dyeing; it should make sure that the substrate has a high and uniform dye uptake and absorbency, is almost completely free of husks, and meets a high quality of whiteness, but must not significantly harm the cellulose.

By boiling off in the presence of alkali, dispersing, and sequestering agents, pretreatment is accomplished. A treatment with a high alkali concentration (causticising or mercerizing) might increase the absorbency even further. Size is eliminated from woven fabrics using the customary techniques before dying.

Typical Recipe for Vat Dyeing:

  • Hydrosulphite, 1.5 g l-1;
  • Stock vat, 62.5 ml l-1;
  • Caustic soda, 5 ml l-1 (27% by weight).
  • Caustic soda (27% by weight), 130 ml l-1;
  •  Hydrosulphite, 60 g l-1;
  • Dispersing agent, 4 g l-1;
  • Wetting agent, 1 g l-1;
  • Indigo Pure (BASF CI Vat Blue 1), 80 g l-1.

Application of Vat Dye:

  • The pigmentation process involves applying the insoluble, dispersed vat color.
  • Using the proper reduction agents, reduce the dyestuff
  • Reduced (leuco) version of the dye dissolving
  • In techniques involving continuous dyeing, the decreased dye is absorbed after the dye has been exhausted from the dyebath.
  • The distribution of dyestuff is consistent through leveling of the dye.
  • Reduced dye is oxidized to create an insoluble pigment state.
  • The final particle dispersion and tone are achieved through soaping.
  • Rinsing is used to neutralize materials and eliminate chemicals.

                   Fig: Basic steps in the application of vat dyes

Vat Dyeing Process:

In general, the vat dyes are not water soluble. However, soluble vat dyes are also offered, albeit they are often more expensive than commonly found indissoluble vat dyes. Vat dyes can be divided into two primary families according to their chemistry: indigo derivatives and anthraquinone derivatives. In comparison to indigo-based dyes, anthraquinone-based vat dyes often have better fastness qualities. Commonly used to create indigo denim with various wash-down and worn-out effects are indigo blue vat dyes.

Basic steps in dyeing with vat dye

With the use of a reducing agent (sodium dithionite) and an alkali (sodium hydroxide), vat dye that is insoluble is transformed into sodium leuco vat anions that are soluble.

Diffusion: fiber infiltration of decreased or solubilized sodium leuco-vat anions

Rinsing: removing surplus reducing agent and alkali from the dyed material.

Oxidation is the process of transforming dye from a vat back into an insoluble form.

When the textile material is soaped, the vat dye molecules absorbed by the fabric are reoriented and join together to form a more crystalline structure.

Properties of Vat Dye:

  1. Vat dyes are organic colorants.
  2. Vat dye cannot be applied directly to fabric since it is insoluble in water.
  3. The insoluble vat colors must be converted by the vatting procedure into soluble form.
  4. PH should be regulated when dyeing protein fibers, but cellulose fibers are the primary target.
  5. These colors have an inadequate tinctorial power. Therefore, the shade depth is little increased by the many-fold increase in concentration.
  6. The fiber has exceptional color stability.
  7. Alkaline media is used for vat dyeing.
  8. Different tones can be found.
  9. The two physical forms of dyes are paste and powder.
  10. The dying process is challenging.
  11. Because dyes are susceptible to light and ester groups are impacted, it is best to avoid exposing dye baths to direct sunlight.
  12. While expensive, dyes have a great leveling property.
  13. The major application of vat dyes is to color cellulosic fibers.
  14. These dyes are capable of producing a wide range of colors.
  15. Vat dye’s washing fastness has a value of 4-5, which is quite good.
  16. Fast rubbing is not a good thing.
  17. Skin disorders can occasionally be caused.
  18. Vat dyes have a finite number of applications in real life.

Structure of Vat Dye:

Vat dyes have a quinoid structure as their distinguishing feature. Since there are no solubilizing groups in the dye’s molecular structure, it is insoluble in water and the majority of solvents in its oxidized form. Since the reduced (leuco) form of the dye is soluble in alkaline solution, the reduction of the quinoid structure is a necessary step in the application of vat dyes. The fiber affine form of the decreased vat dye exhausts from the dyebath (see below Figure).

Fig: Characteristic structures of vat dyes: (a) C.I. Vat Yellow 1 and (b) C.I. Vat Blue 4.

Classification of Vat Dyes:

  1. Sulfur-containing indigo dyes, including both natural and synthetic thio-indigo dyes.
  2. Indigo derivatives, such the brom-indigos, are typically created synthetically rather than directly from indigo.
  3. The many Indanthrene, Marione, and Algol dyes, as well as certain Helindone, are anthraquinone derivatives.
  4. Derivatives of carbazol, of which Hydron Blue is the most notable

Advantage of vat dyes

  • Excellent color fastness characteristics of VAT dyes, such as resistance to fading from light, washing, and other environmental variables, are well known.
  • Given their wide and brilliant color spectrum, VAT dyes are appropriate for a number of applications, including those in textiles, paper, and plastics.
  • Because VAT dyes can withstand chlorine-based bleaches, color brightness and longevity are maintained.
  • They are adaptable for various sorts of fabrics since they may be used on a range of fibers, such as cotton, rayon, and synthetics.
  • Due to their poor water solubility and low risk of water pollution, VAT dyes are typically more environmentally friendly than some other dye kinds.

Disadvantage of vat dyes

  • Due to the requirement for frequent washing, rinsing, and oxidizing stages, the VAT dyeing process can be time- and energy-consuming.
  • The need for specialist facilities and machinery, such as machines for reduction and oxidation, during the dyeing process might raise production costs.
  • Although VAT dyes are adaptable, some fibers and materials may not be compatible with them, which may restrict their use in specific circumstances.
  • Although VAT dyes are typically thought of as being more eco-friendly, the reduction and oxidation processes can produce waste that needs to be properly managed and disposed of.
  • When compared to some other dye kinds, VAT dye manufacture can have greater production costs because it requires intricate chemical synthesis.

Precautions in Vat Dyeing:

The concentration of sodium hydro sulphite and caustic soda must be in the exact amount to lower the dye bath during the entire process in order to provide consistent level dyeing outcomes. Alkali and reducing agents used in excess slow down the dyeing process’ rate of exhaustion. Similar to how insufficient chemical amounts have a negative impact on dyeing, Phenolphthalien paper and Hydro papers are employed to maintain the right equilibrium.

Phenolphthalien Paper: This is white paper that reacts with caustic soda to turn pink.

Vat Yellow Paper: This paper has a gold tint and turns blue-violet if hydro is present in the dye bath.

Conclusion: The stable electron configuration in the chromophore (color-bearing group) of the dye molecule and the presence of multiple benzene rings give the vat dyes high light fastness. Due to the oxidized dye absorbed in the fiber’s aqueous insolubility and the massive vat dye molecules retained within the polymer system, vat dyes exhibit exceptional wash fastness. However, due to the larger number of stages needed in dyeing, vat dyes are typically quite expensive and require more knowledge for their application.

MD. Foysal Kazi
Department of Textile Engineering
BGMEA University of Fashion & Technology

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